This invention relates to oil production, to a floating platform and in particular to a production riser arrangement.
In the production of oil from subsea wells, it is required that the oil be conveyed to the surface, depressurized and degased before it can be pumped for delivery. When a floating production platform is used, it is required that the system include capabilities for disconnecting in emergencies, and the risers passing to the surface must be tensioned to avoid excess stresses caused by loading and ocean currents. Small diameter riser tubing is not readily adaptable to the high tensions required because of its low metal section and because of the high internal pressures existing in the absence of a production choke (throttling valve) at the ocean floor.
Accordingly, prior art multiple well production has included a subsea template incorporating plumbing and control connections for the several wells. It includes subsea trees for each well and a subsea manifold to manifold annulus access and purge riser. A production riser is required to separately transfer oil coming from each well and to provide a common annulus access. Work over risers must be run to perform wire line and tree retrieval operations.
Such a system requires for each well at least five subsea connections between the tubing hanger and the surface and four subsea connections on each control line for straight hydraulic control. It requires the subsea manifold. Several trips are needed to reestablish guidelines for the purpose of servicing a well. The work over riser needs an additional moon pool on the floating platform with additional lifting capabilities.
A floating production facility is described in a paper presented at the 1977 Offshore Technology Conference entitled, "First Floating Production Facility-Argyll," by Messrs. Hammett and Johnson (OTC 2821). The paper describes a multiple well production facility where the various wells are connected to a subsea manifold with the production risers from the wells passing upwardly in annularly-spaced relationship with a central riser. Production trees are located at the wellhead of the vaious wells with reduced-pressure oil being transferred to the subsea manifold. The same paper also suggests the possibility of cluster well drilling, a plurality of wells surrounding the central riser. Production trees are located at each of the wellheads, but their removal requires that the risers are pulled out.